This invention relates generally to tunable lasers and more particularly to enhancing the output characteristics of an external cavity semiconductor laser.
Lasers can be designed so that only a single, narrow wavelength of light is emitted upon activation. Many applications, however, require laser systems that generate variable discrete wavelengths (i.e., tunable lasers). Tunable lasers have been developed and used for years by scientists and engineers to study various optical phenomena. For example, in communication systems, optical fibers exhibit low loss at wavelengths near 1.3 micrometers (.mu.m) and 1.55 .mu.m. Research and development in this area, therefore, require general purpose lasers that can generate laser light in these wavelength ranges.
Conventional tunable lasers, however, such as the external cavity semiconductor laser, color center laser, and the tunable dye laser, do not reliably emit light throughout the entire range of wavelengths required for many present research and development applications (e.g., 1.32.+-.0.1 .mu.m and 1.55.+-.0.1 .mu.m). Tunable dye lasers are difficult to operate, are not commercially available, and the dyes are not stable over time. Color center lasers are available that can be tuned in a small wavelength range around 1.5 .mu.m. However, the color center crystal must be kept at very low temperatures at all times to avoid losing the optical properties that allow it to produce laser radiation. Thus, color center lasers are inconvenient and are not stable over a broad range of environmental conditions. In addition, the color center crystals used in present laser systems cannot generate wavelengths near 1.3 .mu.m. External cavity semiconductor lasers are available around both 1.3 .mu.m and 1.55 .mu.m, but suffer from limited tuning ranges.
In addition to limited tuning range, present tunable lasers also have limited output power. For example, each semiconductor laser has a threshold current which denotes the amount of current required for the laser to efficiently generate light (i.e., lase). Optically pumped lasers also have threshold levels. Below threshold, the light output is very inefficient, and most of the energy from the drive current is lost as heat. Light emission becomes much stronger above the current threshold. This means more of the electrical or optical input energy emerges from the laser as light energy. When the laser threshold is high, more input power is dissipated as heat, which tends to shorten laser operating life and lower the peak output power.
The threshold of a tunable laser changes according to the laser output wavelength. Typically, threshold is higher at the outer edges of the tuning range than at the center of the range. For example, a typical semiconductor laser with a high threshold current near the center of the tuning range usually exhibits a narrow tuning range because the threshold current exceeds maximum allowable operating current relatively close to the center wavelength.
The external cavity laser, as described in U.S. Pat. No. 4,839,308 to Fye, uses a gain medium such as a semiconductor (laser chip). The laser chip has both front and back facets, the back facet having an anti-reflection coating. Light from the laser chip passes through the back facet into an external cavity. The cavity contains a tuning element, such as a prism or grating, that reflects specific laser wavelengths back into the laser chip. This action causes the laser to output selectable wavelengths through the front facet. Thus, the wavelength of light output from the front facet of the laser chip can be controlled by changing the angle of the grating.
U.S. Pat. No. 4,942,583 to Nazarathy et al, and U.S. Pat. No. 5,140,599 to Trutna, Jr. et al, and IEEE Journal of Quantum Electronics, Vol. QE-17, No. 1, January 1981 entitled "Spectral Characteristics of External-Cavity Controlled Semiconductor Lasers" describe tunable external cavity lasers. However, none of the above disclosures address the problems associated with limited tuning range.
Accordingly, a need remains for widely tunable high output power lasers that are stable over varying environmental conditions.